The aim of this project is to obtain structural information on polynucleotides and to provide insights into the factors which control structure, principally through X-ray fiber diffraction and model building studies. Current areas include 1) characterizing the secondary structures induced in DNA by a variety of solvents and conditions, using new sample-handling techniques which give ordered diffraction patterns even in the presence of large amounts of solvent; 2) survey of RNA-DNA hybrid and RNA-RNA duplex structures in the presence of large amounts of solvent; 3) analysis of a unique structure adopted by an RNA-DNA hybrid, poly(rA)-poly(dT) in highly hydrated samples; and 4) collaboration in a joint NMR and X-ray diffraction study of natural and synthetic polynucleotides.